Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory, including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), resistive memory, and flash memory, among others. Types of resistive memory include programmable conductor memory, and resistive random access memory (RRAM), among others.
Memory devices are utilized as non-volatile memory for a wide range of electronic applications in need of high memory densities, high reliability, and data retention without power. Non-volatile memory may be used in, for example, personal computers, portable memory sticks, solid state drives (SSDs), digital cameras, cellular telephones, portable music players such as MP3 players, movie players, and other electronic devices.
RRAM devices include resistive memory cells that store data based on the resistance level of a storage element. The cells can be programmed to a desired state, e.g., corresponding to a particular resistance level, such as by applying sources of energy, such as positive or negative voltages to the cells for a particular duration. Some RRAM cells can be programmed to multiple states such that they can represent, e.g., store, two or more bits of data.
The programmed state of a resistive memory cell may be determined, e.g., read, for example, by sensing current through the selected resistive memory cell responsive to an applied interrogation voltage. The sensed current, which varies based on the resistance level of the memory cell, can indicate the programmed state of the resistive memory cell.
In various instances, arrays of resistive memory cells can include select devices, e.g., transistors and/or diodes, coupled to the resistive storage elements and used to select individual cells for operation. Providing access devices having suitably high current density capability, e.g., 1×105 A/cm2 or higher, while having bidirectional I-V (current-voltage) profile characteristics exhibiting a high selection ratio, e.g., a ratio of current when a cell is selected to current when a cell is unselected, can be challenging.